We are in the process of developing mathematical models to explain the kinetics of the amino acid leucine. However, data analysis has raised further questions regarding the nature of the model, i.e., the domain of validity that will best explain the observed kinetic data. Our immediate concerns are regarding the incorporation of the background levels of the amino acid leucine and its matabolite, KIC (keto-isocaproic acid) when subjects are adapted to two different diets; low leucine (14.1 mg/kg/d) and high leucine (39.2 mg/kg/d). The background levels of these metabolites are crucial to the biological implications of a proposed model as well as the development of a comprehensive and accurate model.